The initial capital of "Zócalo" suggests at first site that it is a place name. It is true, as Helena says, that there is an "El Zócalo" in Mexico, but it is a public square in Mexico City (the Plaza de la Constitución). It can't refer to this.

I can find no relevant place called "El Zócalo", and so I think that despite the capital it refers to the geological term "zócalo", a synonym of "basamento" or "cratón", defined as "Conjunto de rocas que se sitúa bajo una cobertera sedimentaria y se comporta de manera competente durante la deformación cortical" or "Corteza continental generada durante un ciclo anterior al de la secuencia estratigráfica suprayacente, denominada cobertera". The English equivalent is "basement", though it can also be called "bedrock".
REAL ACADEMIA DE CIENCIAS EXACTAS, FÍSICAS Y NATURALES, Glosario de Geología
http://www.ugr.es/~agcasco/personal/rac_geologia/rac.htm#B

"This map shows where the principal rocks are exposed at or near the surface. In detail, the basement (bedrock) is exposed on ridges and mountains. [...]
The deposits have high concentrations of arsenic, and often antimony, as well as gold."
http://www.otago.ac.nz/geology/research/environmental_geolog...

"Anomalous concentrations of lead, copper and antimony mostly occur proximal to outcropping crystalline basement."
http://www.ga.gov.au/ausgeonews/ausgeonews200706/geochemical...


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Note added at 1 hr (2011-12-06 23:23:40 GMT)
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(Sorry: "at first site" in the first line of the explanation is a kind of Freudian slip; it should of course read "at first sight"!)

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Note added at 3 hrs (2011-12-07 01:39:21 GMT)
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"El domo gneísico del Freser (Pirineo Qriental): ¿zócalo precámbrico, intrusión ordovícica o granito herciniano?
The Freser gneissic dome (Eastern Pyrenees): Precambrian basement, Ordovician intrusion, Hercynian granite?"
http://www.sociedadgeologica.es/archivos/geogacetas/Geo36/Ar...

"En el Pirineo existe un zócalo hercínico que aflora mayoritariamente en la Zona Axial"
http://estudiosgeol.revistas.csic.es/index.php/estudiosgeol/...

"The next structural unit, the Hercynian basement thrust sheets, lies just north of the South Pyrenean Central Unit. Here Hercynian basement thrusts are arranged in an antiformal stack known as the Axial Zone. Steeply dipping faults deform the exposed basement rocks in this area (Munoz, 1992)."
http://www.geo.arizona.edu/geo5xx/geo527/Pyrenees/geology.ht...